I took a look at the CE.log and found a lot of error messages indicating problems when sending Email:

2019.05.06 12:27:14 ERROR ce[AWqNFC18pN5NaeWrS4L2][o.s.s.n.e.EmailNotificationChannel] Unable to send email
org.apache.commons.mail.EmailException: Sending the email to the following server failed : mri.EXAMPLE.COM:25
at org.apache.commons.mail.Email.sendMimeMessage(Email.java:1469)
at org.apache.commons.mail.Email.send(Email.java:1496)
at org.sonar.server.notification.email.EmailNotificationChannel.send(EmailNotificationChannel.java:184)
at org.sonar.server.notification.email.EmailNotificationChannel.deliver(EmailNotificationChannel.java:129)
at org.sonar.server.notification.email.EmailNotificationChannel.deliver(EmailNotificationChannel.java:104)
at org.sonar.server.notification.NotificationService.dispatch(NotificationService.java:84)
at org.sonar.server.notification.NotificationService.deliver(NotificationService.java:73)
at org.sonar.server.computation.task.projectanalysis.step.SendIssueNotificationsStep.sendIssueChangeNotification(SendIssueNotificationsStep.java:135)
at org.sonar.server.computation.task.projectanalysis.step.SendIssueNotificationsStep.processIssues(SendIssueNotificationsStep.java:124)
at org.sonar.server.computation.task.projectanalysis.step.SendIssueNotificationsStep.doExecute(SendIssueNotificationsStep.java:100)
at org.sonar.server.computation.task.projectanalysis.step.SendIssueNotificationsStep.execute(SendIssueNotificationsStep.java:90)
at org.sonar.server.computation.task.step.ComputationStepExecutor.executeSteps(ComputationStepExecutor.java:64)
at org.sonar.server.computation.task.step.ComputationStepExecutor.execute(ComputationStepExecutor.java:52)
at org.sonar.server.computation.task.projectanalysis.taskprocessor.ReportTaskProcessor.process(ReportTaskProcessor.java:73)
at org.sonar.ce.taskprocessor.CeWorkerImpl.executeTask(CeWorkerImpl.java:134)
at org.sonar.ce.taskprocessor.CeWorkerImpl.findAndProcessTask(CeWorkerImpl.java:97)
at org.sonar.ce.taskprocessor.CeWorkerImpl.withCustomizedThreadName(CeWorkerImpl.java:81)
at org.sonar.ce.taskprocessor.CeWorkerImpl.call(CeWorkerImpl.java:73)
at org.sonar.ce.taskprocessor.CeWorkerImpl.call(CeWorkerImpl.java:43)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: javax.mail.SendFailedException: Invalid Addresses
at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:2014)
at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1258)
at javax.mail.Transport.send0(Transport.java:255)
at javax.mail.Transport.send(Transport.java:124)
at org.apache.commons.mail.Email.sendMimeMessage(Email.java:1459)
... 26 common frames omitted
Caused by: com.sun.mail.smtp.SMTPAddressFailedException: 550 unrouteable address
at com.sun.mail.smtp.SMTPTransport.rcptTo(SMTPTransport.java:1867)
... 30 common frames omitted

I checked the configuration and was able to send a test email from https://sonarqube.EXAMPLE.COM/admin/settings, so maybe the mail was sent to an inactive former employee. Unfortunately neither the project which triggered the email nor the recipient are logged. I am not in charge of mri.EXAMPLE.COM, so would have to configure a MTA-proxy for more details.

Another error I saw:

2019.05.06 12:00:31 ERROR ce[AWqNBE7MpN5NaeWrS4Lp][o.s.c.t.CeWorkerImpl] Failed to execute task AWqNBE7MpN5NaeWrS4Lp
org.sonar.server.computation.task.projectanalysis.component.VisitException: Visit failed for Component {key=COM.EXAMPLE:MYAPP:src/main/java/COM/EXAMPLE/sse/crmcase/adapter/CrmCaseAdapter.java,type=FILE} located COM.EXAMPLE:MYAPP:src/main/java/COM/EXAMPLE/sse/crmcase/adapter(type=DIRECTORY)->COM.EXAMPLE:MYAPP(type=PROJECT)
at org.sonar.server.computation.task.projectanalysis.component.VisitException.rethrowOrWrap(VisitException.java:44)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visit(PathAwareCrawler.java:52)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visitChildren(PathAwareCrawler.java:87)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visitImpl(PathAwareCrawler.java:70)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visit(PathAwareCrawler.java:50)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visitChildren(PathAwareCrawler.java:87)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visitImpl(PathAwareCrawler.java:70)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visit(PathAwareCrawler.java:50)
at org.sonar.server.computation.task.projectanalysis.step.NewCoverageMeasuresStep.execute(NewCoverageMeasuresStep.java:109)
at org.sonar.server.computation.task.step.ComputationStepExecutor.executeSteps(ComputationStepExecutor.java:64)
at org.sonar.server.computation.task.step.ComputationStepExecutor.execute(ComputationStepExecutor.java:52)
at org.sonar.server.computation.task.projectanalysis.taskprocessor.ReportTaskProcessor.process(ReportTaskProcessor.java:73)
at org.sonar.ce.taskprocessor.CeWorkerImpl.executeTask(CeWorkerImpl.java:134)
at org.sonar.ce.taskprocessor.CeWorkerImpl.findAndProcessTask(CeWorkerImpl.java:97)
at org.sonar.ce.taskprocessor.CeWorkerImpl.withCustomizedThreadName(CeWorkerImpl.java:81)
at org.sonar.ce.taskprocessor.CeWorkerImpl.call(CeWorkerImpl.java:73)
at org.sonar.ce.taskprocessor.CeWorkerImpl.call(CeWorkerImpl.java:43)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.IllegalArgumentException: There's no changeset on line 42
at com.google.common.base.Preconditions.checkArgument(Preconditions.java:145)
at org.sonar.server.computation.task.projectanalysis.scm.ScmInfoImpl.getChangesetForLine(ScmInfoImpl.java:66)
at org.sonar.server.computation.task.projectanalysis.scm.DbScmInfo.getChangesetForLine(DbScmInfo.java:71)
at org.sonar.server.computation.task.projectanalysis.step.NewCoverageMeasuresStep$NewCoverageCounter.initialize(NewCoverageMeasuresStep.java:274)
at org.sonar.server.computation.task.projectanalysis.formula.FormulaExecutorComponentVisitor.processLeaf(FormulaExecutorComponentVisitor.java:165)
at org.sonar.server.computation.task.projectanalysis.formula.FormulaExecutorComponentVisitor.process(FormulaExecutorComponentVisitor.java:142)
at org.sonar.server.computation.task.projectanalysis.formula.FormulaExecutorComponentVisitor.visitFile(FormulaExecutorComponentVisitor.java:122)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visitNode(PathAwareCrawler.java:105)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visitImpl(PathAwareCrawler.java:73)
at org.sonar.server.computation.task.projectanalysis.component.PathAwareCrawler.visit(PathAwareCrawler.java:50)
... 23 common frames omitted
2019.05.06 12:00:31 ERROR ce[AWqNBE7MpN5NaeWrS4Lp][o.s.c.t.CeWorkerImpl] Executed task | project=COM.EXAMPLE:MYAPP | type=REPORT | id=AWqNBE7MpN5NaeWrS4Lp | submitter=XXX | time=429ms